In recent years, the communication system for mobile phones supports multiple communication standards, and each of the communication standards has progressed to a form composed of a plurality of frequency bands. As a part of such mobile phone which performs frequency adjustment and selection, a piezoelectric substrate (“SAW”: Surface Acoustic Wave) device, for example, is used, which is formed with a comb-shaped electrode for exciting the surface acoustic wave on a piezoelectric substrate.
Then, since this surface acoustic wave device is small and has low insertion loss and since it is required to be impervious to unnecessary waves, the piezoelectric materials such as lithium tantalate (LiTaO3, hereinafter also referred to as “LT”) and lithium niobate (LiNbO3, hereinafter, also referred to as “LN”) are used to make the surface acoustic wave device. In particular, the communication standard for the fourth generation mobile phones specifies in many cases a relatively narrow frequency band interval between send and receive communications and a relatively large band width; however, since the properties of the conventional materials used to make the surface acoustic wave device are variable with temperature, so that a problem that has occurred is that the frequency selection area is displaced with a result that the filter and duplexer functions are interfered. Therefore, materials that are less variable with respect to temperature, and has a relatively large band width are craved for as the material to make the surface acoustic wave device.
In the manufacturing process of the surface acoustic wave device, there are more than one steps where the raw material is subjecting to a temperature of 100-300° C., so that if there is pyroelectricity in the surface acoustic wave device material, the material can be charged at higher than 1 KV and discharge electricity. This discharge is not desirable because the manufacturing yield of the surface acoustic wave device decreases. Further, even if the pyroelectricity is so weak that the charging of the surface acoustic wave device material decreases with time, this is not desirable yet, because the noise is generated in the electrode of the surface acoustic wave device by the temperature change.
On the other hand, IP Publication 1 describes that the stoichiometric composition LT obtained mainly by the vapor phase method, using copper as the electrode material, is desirable since it causes it more difficult for the breakdown mode to occur in which the device is destroyed at the moment when the higher power is input to the IDT electrode. In IP Publication 2 there is a detailed description relating to the stoichiometric composition LT which is obtained by the vapor phase method. Further, IP Publication 5 and non-IP Publication 2 also contain a report saying that the use of a denatured LT, in which Li is enriched uniformly in the direction of thickness by means of vapor phase equilibrium method, as the material to make the surface acoustic wave device, is desirable since the frequency temperature behavior of the device is improved.
However, it was found that in the methods disclosed in these IP publications favorable results are not necessarily obtained. In particular, according to the method described in IP Publication 5, it takes no less than 60 hours at a high temperature of about 1300° C. for the wafer to be processed, because the processing is conducted in a gas phase, and as the result the high manufacturing temperature, consequent substantial warpage of the wafer, and high rate of crack generation lead to poor productivity and overly pushed-up price for a surface acoustic wave device material. Furthermore, the vapor pressure of Li2O is so low that depending on the distance from the Li source, the sample being denatured undergoes uneven denaturing whereby the properties similarly show uneven distribution and hence the industrialization of it requires significant improvement.
Further, IP Publication 5 teaches about a manufacturing condition comprising a substrate thickness of 0.5 mm t and a treatment temperature of 1200° C.-1350° C.; however this is none other than a time-honored conventional manufacturing procedure, and this thickness is far greater than that commonly required of the substrate for the surface acoustic wave device. It is conceivable to make the substrate thinner until it obtains a desired thickness after the vapor phase treatment; however since the diffusion of Li has caused some deformation in the substrate so that the rate of cracking during the thinning procedure is increased, and what is more, such operation adds to the manufacturing cost and it is economically irrational and a waste of material resulting from halving the 0.5-mm-thick substrate to 0.25-mm-thickness—an unjustifiable cause for heightening the price.
Moreover, in the course of the investigation by the present inventors with regard to the lithium tantalate single crystal substrate for surface acoustic wave element described in IP Publication 5, they found that the substrate had weak pyroelectricity, and consequently they conducted procedures so as to remove this pyroelectricity, and in one example they adopted the method taught in the description of IP Publication 6, but it was impossible to completely remove the pyroelectric effect.
Then, IP Publication 3 discloses a manufacturing method wherein LiNbO3 and LiTaO3 are subjected to a proton exchange treatment to thereby create a refractive index distribution in the surface layers of LiNbO3 and LiTaO3 or the like. But once the proton exchange is effected, the piezoelectric property of the materials such as LiNbO3 and LiTaO3 is impaired and as the result there occurs a problem that they cannot be used as a material for surface acoustic wave device.
In addition, Non-IP Publication 1 teaches that a 38.5° rotation Y cut LiTO3 having a fixed ratio composition (hereinafter also referred to as “stoichiometry composition LT or SLT”) which is obtained by the pulling method employing a double crucible, is more preferable compared to the one having a melt composition of LiTaO3 (hereinafter also referred to as “congruent composition LT, or CLT) which is obtained by the usual pulling method wherein the compositions of the melt and that of the raised crystal are identical to each other, for the reason that the electromechanical coupling factor of the former is 20% higher than the latter. However, in the case of the LT of non-IP Publication 1, the pulling speed to obtain the SLT has to be one order lower than that with the usual pulling method, so that the cost of the SLT becomes so high that it is difficult to use it in the application for surface acoustic wave device.